Secondary batteries that can be charged and discharged repeatedly are used in electronic devices such as cellular phones, personal computers, and digital cameras. Electricity accumulation systems for use against momentary voltage drops, electricity accumulation systems mounted on electric vehicles and hybrid electric vehicles, and so forth require an electricity accumulation device that has a high energy capacity and that can be charged and discharged quickly. Examples of such an electricity accumulation device include lithium ion secondary batteries and electric double layer capacitors.
In general, the electricity accumulation device such as a secondary battery and a capacitor includes an electrode laminate in which a positive electrode and a negative electrode are laminated via a separator. The electrode laminate is sealed in an outer jacket material such as a metal case or a laminate film together with an electrolyte solution. In the electricity accumulation device, the electrolyte solution may be decomposed to generate a gas through overcharge or repeated charge and discharge. This may reduce the capacity of the device, or may shorten the life of the device. If the internal pressure is raised by the generated gas, the outer jacket material may be expanded to impair safety. Therefore, there is adopted a scheme in which the temperature, current, voltage, etc. of the electricity accumulation device are measured to stop charge when such values exceed respective prescribed values, for example. A secondary battery including a sensor that senses expansion of the outer jacket material is proposed (see Patent Documents 1 to 3).